1. Field of the Invention
The present invention relates to an IP (Internet Protocol) communication apparatus and an IP communication method of the apparatus and system, the IP communication apparatus being connected to a wide-area network via a relay apparatus that has a NAT (Network Address Translation) function.
2. Description of Related Art
Conventionally, an IP communication apparatus (IP telephone and the like) connected to a LAN (Local Area Network) at office or home is generally connected to a WAN (Wide-Area Network) via a predetermined relay apparatus (router and the like). There is a relay apparatus installed with a NAT function for transparently performing conversion between a private IP address, which is valid only on a LAN, and a global IP address, which allows access to an external WAN. Such a relay apparatus has advantages, including increased flexibility in providing IP addresses to apparatuses connected to the LAN and improved security on the LAN.
However, to access an IP telephone service using such an IP communication apparatus connected to the WAN via the relay apparatus that has the NAT function, for example, SIP (Session Initiation Protocol) or the like, which is used as a call control protocol, adds a private IP address and port number to a data portion of an IP packet for communication. Thus, the NAT function, which converts only an IP address in a header portion of an IP packet, cannot convert the IP address in the data portion. In addition, even when adding a global IP address to the data portion of the IP packet, the IP communication apparatus has a problem where the apparatus cannot recognize the global address and port number information. As a result, a problem has arisen where the relay apparatus blocks delivery of a communication packet from the WAN side to the LAN side, which is a commonly-called NAT traversal problem.
To address such problems, methods are known that use protocols, including UPnP (Universal Plug and Play) and STUN (Simple Traversal of User Datagram Protocol). In the UPnP method, for instance, an IP communication apparatus that has a UPnP function is capable of obtaining information, including a global IP address, port number and the like, from a relay apparatus that supports UPnP IGD (Internet Gateway Device). The method is, however, based on the premise that the IP communication apparatus, relay apparatus, and the like support UPnP. Thus, the method is difficult to be applied to an apparatus and the like that do not have the UPnP function. Further, when a plurality of relay apparatuses exist, even provided with the UPnP function, it is not easy to grasp locations of all the relay apparatuses, and it is thus difficult to obtain necessary information, such as global IP addresses, port numbers, and the like. In the STUN method, meanwhile, a STUN server can be installed so as to provide necessary information (a global IP address, port number, NAT type, and the like) in response to a request from an IP communication apparatus on a network. Thereby, the method can be applied regardless of availability of the UPnP function. Even the STUN method, however, has a certain limit depending on an applied environment.
Conventional technology to achieve NAT traversal using UPnP and STUN is known in an information communication system, for example, where a plurality of terminal apparatuses are connected to different routers and perform P2P (Peer to Peer) communication over the Internet. When a connected router has the UPnP function, the terminal apparatus obtains a global IP address and port number based on a UPnP protocol, and registers the obtained information with an information controller on the Internet as exchange information. When a connected router has no UPnP function, on the other hand, the terminal apparatus obtains a global IP address and port number based on a STUN protocol, and registers the obtained information with the information controller as exchange information. For communication between the terminal apparatuses, the apparatuses mutually obtain the exchange information of the apparatuses to communicate with and achieves NAT traversing communication using the obtained exchange information (see Related Art 1).    [Related Art 1] Japanese Patent Laid-open Publication No. 2005-151142
The above-described NAT is categorized into four types according to its operational behavior: full cone, restricted cone, port restricted cone, and symmetric. Symmetric NAT herein correlates a private IP address and port number of a source terminal with a plurality of pairs of a global IP address and port number for different addresses of destination external apparatuses. With the conventional technology described in Related Art 1, therefore, when at least one terminal apparatus employs symmetric NAT, a global IP address and port number obtained based on the STUN protocol are different by destination. Thus, it is difficult to ensure NAT traversal with the technology as it fails to properly determine the port number, unless the NAT correlation method is known in advance or other special arrangements are made.